The present invention relates to a vibration sensor using FM technology. More particularly the invention relates to a vibration sensor using a strain-sensitive oscillator in which resonant microbeams are driven and sensed by a single mulimode optical fiber.
Vibration sensors can be useful for monitoring the condition of rotating machinery, where overheating or excessive vibration could indicate excessive loading, inadequate lubrication, or bearing wear. Commercial vibration sensors use a piezoelectric ceramic strain transducer attached to a metallic proof mass in order to respond to an externally imposed acceleration.
It is also possible to use a MEMS accelerometer with a capacitive output such as the one made by Analog Devices. These devices provide an acceleration output directly and do not require FM demodulation.
However, alternative means for measuring vibration, which would have decreased cost, easier manufacturing techniques, more reliability and additional features, are always desired. It would be important to develop new sensors which would overcome some of the shortcomings of current sensors, particularly by offering the many advantages of fiber optic interconnections. Of particular interest are sensors which have immunity from EMI, are intrinsically safe, and permit operation at much higher temperatures than the prior art.
Also, since vibration is often measured or monitored to determine or anticipate defects and/or machine wear, it would be desirable to measure the temperature of the object being monitored since an increase in heat often indicates increased friction or stress that may in time lead to failure of the object.
Accordingly, it would be of great advantage in the art if an improved vibration sensor could be provided.
It would be another great advance in the art if the vibration sensor would also permit measurement of other physical properties such as the temperature of the vibrating element.
Other advantages will appear hereinafter.
It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, the present invention provides a sensor device for detecting vibration. For the first time, a strain-sensitive resonator is used as a vibration sensor. The vibration sensor of the present invention makes use of acceleration-sensitive devices having their output in the form of a frequency that depends on the acceleration to which they are subjected Such devices are known in the art as resonant acceleration sensors, or resonant accelerometers, which belong to the broad class of resonant sensors. Resonant sensors make use of a strain-sensitive resonant element, or resonator, preferably having high mechanical Q so that its resonant frequency is well defined. Examples of srain-sensitive resonators are clamped-clamped bems or thin diapragms. The resonator is attached to or built on to a sensitive structure so that an externally imposed variable such as presure, temperature or acceleration acts to change the natural vibration frequency of th resonator. In a common embodiment of a resonant sensor, the resonator is used to determine the frequency of an oscillator which generates an AC output at a frequency characteristic of the resonator. An external power source is required to maintain the vibration of the resonator and to provide an oscillator output to an external load or detector. The extermal power source can be electrical or optical and the oscillator output can be either an AC electrical signal or it can be in the form of intensity modulated light.
In a resonant accelerometer the resonator is attached to or is part of a structure that deforms or bends in resonse to an external acceleration. An example is a flexible member, or flexure, attached at one end to a more massive member called a proof mass, and at the other end to an external frame or structure which is subjected to acceleration. A resonant element such as a clamped-clamped microbeam is mounted on the flexure. When subjected to an external acceleration, the inertia of the proof mass causes themotion of the proof mass to lag the motion of the frame and causing bending of the flexure which in turn changes the tension in the microbeam and therefore its resonant frequency much as the tightening of a string of a musical instrument change its pitch
In particular, the invention makes use of a resonant accelerometer in which vibration of the structure causes FM modulation of the oscillator output which can be detected by an FM discriminator circuit In a preferred embodiment, the oscillator is powered optically by means of an optical fiber and the output is in the form of light with an intensity modulated at the frequency of the strain-sensitive resonator.
The device of this invention include""s a light source, such as a laser, which is reflected off the strain-sensitive oscillator, returning as frequency modulated light and converted to an electrical signal by a photodetector. This elecrical signal is then demodulated in a FM discriminator to produce an output signal representative of the vibration.
A frequency meter may also be used to determine or count the average number of cycles per unit time to provide a second signal responsive of the temperature of the oscillator.
The laser light and the frequency modulated light reflected from the oscillator are transmitted from the laser light source to the oscillator and back to the frequency modulated discriminator by optical fibers. A pair of optical fibers may be used, or the same fiber may both transmit the laser light to the oscillator and capture the reflected frequency modulated light for detection and transmission to the demodulator.
The preferred oscillator includes a microchip having a microbeam mounted on a thin silicon cantilever such that deflection of the beam perpendicular to the plane of the microchip changes the tension in the microbeam to change its resonant frequency. Also preferred is a microbeam including a thin metal deposit to create a bimorph structure. Other strain-sensitive oscillators may be used as well.
For a more complete understanding of the invention, reference is hereby made to the drawings, in which:
FIG. 1 is a block diagram schematically illustrating the device of the present invention in its most basic form;
FIG. 2 is a block diagram schematically illustrating the device of the present invention in an optical form;
FIG. 3 is a schematic cross sectional view of one embodiment of the present invention, showing a polysilicon microbeam fabricated on a silicon cantilever paddle, all in accordance with the invention;
FIG. 4 is a graph showing a spectrum of an amplified signal showing a vibration having been detected as an FM modulated signal;
FIG. 5 is a graph showing a spectrum of a signal showing a demodulated signal indicating a vibration having been identified;
FIG. 6 is a graph showing a spectrum from a detector circuit;
FIGS. 7a and 7b are alternative versions of two accelerometer designs; and
FIG. 8 is a graph showing the calculated primary and cross axis sensitivity versus suspension natural frequencies in an accelerometer.